Complementary metal oxide semiconductor (CMOS) devices, such as metal oxide semiconductor field-effect transistors (MOSFETs), are commonly used in the fabrication of ultra-large scale integrated (ULSI) devices. The continuing trend is to reduce the size of the devices and to lower the power consumption requirements. Size reduction of the MOSFETs has enabled the continued improvement in speed performance, density, and cost per unit function of integrated circuits.
FIG. 1 illustrates one type of a MOSFET formed on a substrate 110. The MOSFET generally has source/drain regions 112 and gate electrodes 116. A channel 118 is formed between the source/drain regions 112. The gate electrode 116 is formed on a dielectric layer 120. Spacers 122 are formed on each side of the gate electrode 116, and contact pads or silicide pads 124 are formed on the source/drain regions 112 and the gate electrodes 116. The source/drain regions 112 and/or the contact pads 124 may be raised. Isolation trenches 126 may be used to isolate the MOSFETs from each other and other devices (not shown).
The contact pads 124 provide reduced contact resistance and are frequently formed of a metal silicide. Furthermore, the contact pad 124 on the gate electrode 116 is generally formed in the same process steps as the contact pad 124 on the source/drain regions 112, and thus, has the same characteristics. Many times, however, it is desirable that the silicided portions of the source/drain regions 112 exhibit different operating characteristics.
Furthermore, as the size of semiconductor devices are reduced, it is desirable to use a metal gate electrode, such as a fully silicided gate electrode, to further reduce resistance. Attempts have been made to fabricate a highly conductive gate electrode by performing a silicidation process on the poly-cystalline semiconductor gate electrode, which is frequently a poly-silicon (poly-Si) material or poly-SiGe material. Generally, the silicidation reaction converts the poly-semiconductor material to a highly conductive silicide. One method of fabricating a semiconductor device having a fully silicided gate electrode is described in U.S. Pat. No. 6,475,874 entitled, “Damascene NiSi Metal Gate High-K Transistor,” which is incorporated herein by reference.
Often, however a different type of metal is desired or a different amount of silicidation is desired in order to create varying work functions dependent upon the device and its characteristics. Thus, there is a need for a dual silicided structure in which characteristics may be tuned or optimized for a particular application.